Zoom lens designs are based on the property that the power of an optical system consisting of at least two lens groups can be varied by changing the distance between the groups. The lens capabilities depend on the number of moving groups in the system. This is discussed by W. J. Smith in “Modern Optical Engineering” McGraw-Hill, 1990. In any zoom system, at least two lens groups must be moved with respect to each other in order to have a variable focal length system and a fixed image plane position.
The complexity of a lens mechanical mount, or cam, is determined by the number of moving groups within the zoom lens. An example of a simple cam with two grooves is shown in W. J. Smith, FIG. 9.31, p. 276.
More moving optical groups may be required if other optical system characteristics are needed such as quality imaging over a range of object distances with large zoom power, or if the entrance and exit pupil locations need to be fixed. More elements within each group are often required to compensate for aberrations, as is the case with any traditional lens system.
Most of the modem miniature zoom lenses are composed of two groups of negative and positive powers. Such systems then have small size but a long back focal length, which is a serious drawback. For minimization purposes, these lens groups are further divided into subgroups that move independently to extend the zooming range and to attempt to minimize the overall size of the system. See, for example, U.S. Pat. No. 4,936,661 granted to E. I. Betensky, et al Jun. 26, 1990, U.S. Pat. No. 5,270,861 and U.S. Pat. No. 5,270,867 both granted to L. R. Estelle on Dec. 14, 1993. A two-element zoom system with negative and positive plastic elements is discussed in U.S. Pat. No. 5,473,473 granted to L. R. Estelle on Dec. 5, 1995. This is a 35 mm format lens with a speed of F/11 in the wide-angle position.
There is a continuing need for a small, compact, and inexpensive zoom lens system.